With 100 percent of the state in drought and two-thirds of California classified as in exceptional drought, the most severe category, Californians are rapidly depleting underground reserves of water that took thousands of years to accumulate. In a rainy year, the state relies on groundwater for about a third of its water supply; this year the figure is approaching 60 percent as rivers and reservoirs dry up.

Famiglietti predicts the state could drain all its groundwater in as soon as 60 years, given the current rate of depletion.

But no one really knows how much is left—California has never measured the volume of groundwater it pumps, allowing property owners who drill a well to take as much as they want.

Legislation passed by California lawmakers on Friday changes that dynamic. Well owners in the state’s most stressed basins will have to monitor how much water they withdraw and report the data to a local Groundwater Sustainability Agency. The GSAs are charged with ensuring long-term groundwater supplies and will have the power to “control groundwater extractions by regulating, limiting, or suspending extractions from individual groundwater wells or extractions from groundwater wells in the aggregate,” according to the legislation.

California Gov. Jerry Brown has supported the package of bills and is expected to sign them into law, making California the last Western state to manage its groundwater supply.

If local officials don’t manage their water basins sustainably, the legislation allows state regulators to intervene.

“I think the legislation is a step in the right direction,” Stephanie Castle, who until recently was a water resources specialist at UC Irvine, said in an email. “Developing towns in California will no longer be able to assume that groundwater will always be readily available and will be forced to determine whether or not new development will be sustained in the future in terms of water supply.”

The question is, How much more of a groundwater deficit will California dig itself into before the law takes effect?

“We are already observing some local regions (Paso Robles, for example) that have already dried up some of their wells,” said Castle. “It really depends on the amount of groundwater each region is using, and how much that region has left.”

That won’t be known for certain until data on groundwater pumping begins to be collected in 2020. In the meantime, technology can provide some answers.

A satellite program called the Gravity Recovery and Climate Experiment measures tiny changes in an area’s gravitational pull to determine its groundwater capacity. Scientists recently analyzed data collected by GRACE to determine that the Colorado River Basin, which supplies water to 40 million people, lost 65 cubic kilometers of water—that’s 17.3 trillion gallons—between 2004 and 2013. More than two-thirds of the loss was to groundwater.

Replenishing that supply won’t happen anytime soon. A recent study predicted as much as a 50 percent chance that the Southwest United States will be hit with a 35-year drought this century, and a 90 percent chance of a decade-long dry spell.

“With the fact that we are actually in a ‘wet’ period now, based on tree ring records, and the prediction of prolonged droughts in the future, groundwater management in California is definitely not a gamble—it is a necessity,” Castle said.

Reporting and regulating groundwater withdrawals may be only the beginning if California is to transform how it manages its water.

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